41．0n the rare occasion when a fine piece ofsculpture was desired，Americansturned to foreign sculptors，as in the1770's when the cities of New York and Charleston, South Carolina，commissioned the Englishman Joseph Wilton to make marble statues ofWilliam Pitt．

(句子主干是Americans turned to foreign sculptors；两个when都引导定语从句：occasionwhen…，1770's when…；as(in the 1770's)的意思是“正如”)

在少数情况下，当需要一件精美的雕刻品时，就像1770年纽约与南卡罗来纳的查尔斯顿城委托英国人约瑟夫·威尔顿制造威廉·皮特的大理石雕像一样，美国人求助于外国人。

分句1：0n the rareoccasion when a fine piece of sculpture was desired

分句2：Americans turned to foreignsculptors

分句3：as in the 1770's when the cities ofNew York and Charleston, South Carolina，commissioned the Englishman Joseph Wilton to make marble statues ofWilliam Pitt．

分句2是整个长句子的主句

分句1是分句2的定语从句

分句3是分句2的定语从句

1和2嵌套

2和3并列